The invention is in the field of hybrid junctions and specifically relates to four port junctions known as magic tees.
A common type of waveguide hybrid junction known as the magic tee is a four port microwave device comprised of electrically coupled waveguide sections physically disposed about a plane of symmetry through one of the sections. That is, a first section termed the H-plane arm and two additional sections, termed the two collinear side arms, are joined to form an H-plane junction between the H-plane arm and the two collinear arms. These three sections are disposed in the shape of a tee. A fourth section, termed the E-plane arm is joined to the tee forming an E-plane junction between the H-plane arm and the E-plane arm. The collinear side arms and the E-plane arm are also located, relative to each other, in the shape of a tee.
When properly designed, the hybrid junction just described is electrically symmetrical and appears to possess what has been called magical properties; thus, the name magic tee. These properties include equal power division into the two collinear side arms (provided they are terminated in matched loads) when power is applied to either the H-plane arm or the E-plane arm. Significantly, with matched loads in the collinear side arms there is no coupling between the E-plane arm and the H-plane arm. Thus, when the signal is applied to the H-plane arm no signal appears in the E-plane arm and vice versa.
When the input signal is fed to the H-plane arm the electric field in the two collinear arms are in phase at points equal distances from the center of the junction. As a result, the vector sum of signals applied to the two collinear arms is produced in the H-plane arm. Because of this property, the H-plane arm is considered as being connected in shunt or parallel with the collinear side arms. If power is supplied to the E-plane arm, the electric field in the two collinear arms will be 180.degree. out of phase at points equal distances from the center of the junction. The vector difference of the signals applied to the two collinear arms is seen in the E-plane arm. The E-plane arm is, therefore, viewed as the series arm, meaning that the E-plane arm appears to be connected in series with the two collinear arms.
The impedance looking into the H-plane and the E-plane arms with properly matched loads in the two collinear side arms is not matched to the input waveguides. If, by addition of matching structures, these impedances are made to match the input waveguides the device will possess the additional quality of balance and reflection of an input signal to either the H-plane arm or E-plane arm will be minimized. Matching of the H-plane arm and the E-plane arm is conventionally accomplished by the addition of matching structures such as metal diaphragms. However, as the voltage standing wave ratio that must be matched is generally high, the bandwidth is small. To improve bandwidth it is known to place the matching structures at the heart of the junction. A typical matching structure for matching the impedance looking into the H-plane arm to the input waveguide involves centrally locating a metallic post in the junction. The optimum length and position of this post is determined experimentally. In the past there was little concern with post diameter. The post diameter affects the maximum power which can be handled by the magic tee. The maximum power capability is directly related to the breakdown voltage between the post and the walls of the waveguide section forming the E-plane arm. The breakdown voltage is the maximum voltage which can be tolerated before arcing occurs across the gap between the post and E-plane arm walls. It was believed that the breakdown voltage increased in direct proportion to the gap size. That is, it was thought that to increase breakdown voltage and thus the power handling capacity of the junction, one need only reduce the post diameter, thereby increasing the space or gap between the walls of the E-plane arm and the post. However, even with relatively thin posts, the magic tee remained a low power device for arcing between the tip of the post and the walls of the E-plane arm limited the power that could be applied to the junction.